4,4'-Methylenebis (2-chloroaniline) is a well-known and valuable curing agent for polyurethane prepolymers because of the properties it imparts to the polyurethane product. In commercial practice, for example in the preparation of urethane castings, 4,4'-methylene-bis (2-chloroaniline) is usually mixed as a molten fluid into a heated polyurethane prepolymer. Melting the solid diamine is inconvenient and extremely hazardous since 4,4'-methylenebis (2-chloroaniline) has recently been classified as a suspected carcinogen and its use has been severely curtailed. Furthermore, the addition of molten 4,4'-methylenebis (2-chloroaniline) to the preheated polyurethane prepolymer results in a polyurethane which has a short pot life. That is, there is insufficient time in many cases for mixing the molten diamine with the polyurethane prepolymer and pouring of the mixture into molds before the mixture becomes unpourable.
To overcome the difficulties associated with incorporating solid 4,4'-methylenebis (2-chloroaniline) into a polyurethane prepolymer, there have been proposed a variety of techniques for curing polyurethane prepolymers with compositions containing 4,4'-methylenebis (2-chloroaniline) to obtain polyurethane compositions with satisfactory pot life and good physical properties. Some of these techniques are illustrated in U.S. Pat. Nos. 3,718,619 and 3,718,624. The synthesis and use of compounds analogous to 4,4'-methylenebis (2-chloroaniline) is described in U.S. Pat. Nos. 3,408,301 and 3,728,310. The use of bis(aminoarylene) sulfones, such as bis(aminophenylene) sulfone to cure urethane prepolymers is described in U.S. Pat. No. 3,355,435.